nsqd: tighten up stats related locking even more
nsqd: tighten up stats related locking even more

just as we don't need to lock the whole instance for the lifetime
of the stats gathering, we don't need to lock the topic for the
lifetime of the channel stats gathering

bitly/nsq,coderhaoxin/nsq,absolute8511/nsq,nsqio/nsq,jehiah/nsq,mreiferson/nsq,jlr52/nsq,mreiferson/nsq,jehiah/nsq,sparklxb/nsq,sparklxb/nsq,szqh97/nsq,jehiah/nsq,nsqio/nsq,sparklxb/nsq,absolute8511/nsq,absolute8511/nsq,ploxiln/nsq,bitly/nsq,rpau/nsq,coderhaoxin/nsq,rpau/nsq,mreiferson/nsq,rpau/nsq,jlr52/nsq,MadBomber/nsq,ploxiln/nsq,coderhaoxin/nsq,MadBomber/nsq,jlr52/nsq,szqh97/nsq,MadBomber/nsq,mreiferson/nsq,absolute8511/nsq,MadBomber/nsq,sparklxb/nsq,MadBomber/nsq,nsqio/nsq,ploxiln/nsq,jehiah/nsq,mreiferson/nsq,szqh97/nsq,coderhaoxin/nsq,szqh97/nsq,bitly/nsq,jlr52/nsq,bitly/nsq,ploxiln/nsq,szqh97/nsq,absolute8511/nsq,jehiah/nsq,sparklxb/nsq,coderhaoxin/nsq,rpau/nsq,nsqio/nsq,jlr52/nsq,rpau/nsq,ploxiln/nsq,nsqio/nsq,bitly/nsq